Daniel Glaser, assistant secretary for terrorist financing at the U.S. Treasury Department, said the air strikes had hit the group’s ability to extract, refine and transport oil from the territory it controlled in Iraq and Syria.

“When you look at difficulties that we know that they are having with respect to the transport, with respect to the extraction, I think it’s fair to say they are no longer able to make money the way they used to be able to,” Glaser told a conference in London.

“(IS) has cut salaries to its fighters in Raqqa by up to 50 percent,” he added.